Organic microgel system for 3d printing of silicone structures

ABSTRACT

An organic microgel system as support material for 3D printing of soft materials such as silicone and methods for manufacturing and using the organic microgel system are disclosed. In some embodiments, the organic microgel system comprises a plurality of microgel particles formed by blending a di-block copolymer and a tri-block copolymer in an organic solvent. The organic microgel system may allow high precision 3D printing of silicone objects with complex shapes.

RELATED APPLICATIONS

This Application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Application Ser. No. 62/382,652, entitled “ORGANIC MICROGELSYSTEM FOR 3D PRINTING OF SILICONE STRUCTURES” filed on Sep. 1, 2016,which is herein incorporated by reference in its entirety.

GOVERNMENT SUPPORT

This invention was made with Government support under DMR1352043 awardedby the National Science Foundation. The Government has certain rights inthis invention.

FIELD

Disclosed embodiments relate to materials for supporting silicone-basedink in a three-dimensional printing operation and methods formanufacturing the same.

BACKGROUND

Three dimensional printing (3D printing) is a novel additivemanufacturing technique in industrial and consumer applications.

Objects made of soft materials, such as silicone elastomer, enjoy a widerange of applications in biomedical diagnostics, therapeutics as well asconsumer electronics. Silicone elastomer enables properties such as lowelastic modulus, high extensibility and toughness, excellent thermal andoxidative stability, and chemical inertness. However, it is difficult orimpractical to form many objects of silicone elastomer usingconventional 3D printing techniques. Conventional approaches are notsuitable for efficiently forming objects of soft materials with complexshapes and high resolution.

The inventors have recognized and appreciated efficient manufacturingmethods that can be applied in existing applications as well as toenable new applications for 3D printing in forming silicone-basedobjects.

SUMMARY

In some embodiments, a method of manufacturing a three-dimensionalsilicone structure is disclosed. The method comprises injecting inkmaterial comprising silicone into a support material and displacing thesupport material with the ink material, and curing the silicone materialwhile retaining the ink material with the support material. The inkmaterial is injected in a pattern conforming to the three-dimensionalstructure and the support material comprises an organic solvent and ablock copolymer such that it is immiscible with the ink material.

In some embodiments, a support material for supporting silicone-basedink in a 3D printing operation is disclosed. The support materialcomprises a plurality of microgel particles. Each of the plurality ofmicrogel particles comprises a crosslinked polymer network. Thecrosslinked polymer network comprises a plurality of tri-block copolymermolecules; a plurality of di-block co-polymer molecules; and an organicsolvent. The plurality of microgel particles are swollen in the organicsolvent.

In some embodiments, a method of manufacturing a support material forsupporting silicone-based ink in a 3D printing operation is disclosed.The method comprises blending a di-block copolymer and a tri-blockcopolymer in an organic solvent; heating the solvent mixture to above afirst temperature and cooling the solvent mixture from the firsttemperature to below the first temperature to form a plurality ofmicrogel particles.

It should be appreciated that the foregoing concepts, and additionalconcepts discussed below, may be arranged in any suitable combination,as the present disclosure is not limited in this respect. Further, otheradvantages and novel features of the present disclosure will becomeapparent from the following detailed description of various non-limitingembodiments when considered in conjunction with the accompanyingfigures.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures may be represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 is a schematic representation of one embodiment of an apparatusfor printing an ink material in a support material;

FIG. 2 is a schematic diagram depicting printing of ink material in asupport material comprising di-block copolymers and another supportmaterial comprising tri-block copolymers;

FIG. 3 is a schematic diagram depicting printing of ink material in asupport material comprising a blend of di-block copolymer SEP andtri-block copolymer SEBS in mineral oil;

FIG. 4 is a data plot of measured shear modulus G′ and G″ as a functionof oscillatory stress frequency for microgels prepared according to someexemplary embodiments;

FIG. 5 is a data plot of measured shear stress σ as a function of shearrate c for microgels prepared according to some exemplary embodiments;

FIG. 6 is a data plot of measured shear rate c as a function of recoverytime after yielding for microgels prepared according to some exemplaryembodiments;

FIG. 7 is a phase contrast microscopy image of a microgel preparedaccording to an exemplary embodiment and diluted in mineral oil;

FIG. 8 is a data plot of measured small angle X-ray scattering (SAXS)intensity as a function of wave vector q for microgels preparedaccording to some exemplary embodiments;

FIG. 9 is a confocal microscopy image of printed silicone featuresaccording to some embodiments;

FIG. 10 is a data plot of measured printed feature area A as a functionof material flow rate and tangential nozzle velocity;

FIG. 11A is a photograph showing measurement of interfacial tensionusing the pendant drop method;

FIGS. 11B-11E are data charts showing interfacial tension measurementsfor the interfaces measured using the pendant drop method, according tosome embodiments;

FIGS. 12A-12C are data plots showing results from scanning white lightinterferometry measurements;

FIG. 12D is a scanning electron microscopy image of the cross-section ofa printed silicone structure;

FIG. 12E is a scanning electron microscopy image of a 3D printed“dog-bone” part designed for testing mechanical strength;

FIG. 12F is a dataplot showing extensional stress-strain tests ondog-bone samples;

FIGS. 13A-13G shows photographs of 3D printed silicone structures usingmicrogel system as support material according to some embodiments.

DETAILED DESCRIPTION

The inventors have recognized and appreciated that a new supportmaterial for 3D printing of soft materials such as silicone may beprovided using the materials and methods described herein. The inventorshave recognized and appreciated that using an organic microgel system assupport material for 3D printing of silicone as described herein mayallow for high precision printing of silicone objects with complexshapes.

The inventors have recognized and appreciated that creating a supportmaterial using the materials and methods described herein may allow for3D printing with ink material comprising silicone elastomers withoutissues such as instabilities related to ink material sag and interfacialtension. The inventors have recognized and appreciated that during 3Dprinting of soft ink materials such as silicone, it is desirable to usea support material that provides sufficient mechanical support for theprinted structure to reduce ink material sag and dispersion in order toavoid unstable structures while the silicone-based ink is cured. Theinventors have also recognized and appreciated that a low interfacialtension between the immiscible ink material and support material isdesired in order to avoid unstable breakup of the printed structures.

Further, the inventors have recognized and appreciated formulations ofsupport material that may achieve some or all of these objectives. Insome embodiments, the microgel system comprises organic solvents such asmineral oil. The inventors have recognized and appreciated that theinterfacial tension between silicone and organic solvents is low, whichis beneficial for precise 3D printing of silicone structures, such thata microgel system using an organic solvent is well suited for 3Dprinting of silicone-based objects.

In some embodiments, a microgel system is provided as the supportmaterial with desired rheological properties including low modulus, lowyield stress, localized yielding behavior, and spontaneous re-flow afteryielding and short thixotropic time that may overcome instability issuesin silicone 3D printing and allow for precision 3D printing of inkmaterials immiscible in the support material. In some embodiments,packed microgels provide a self-healing environment, allowing a printingnozzle to transverse the same region repeatedly while simultaneouslysupporting the structure.

In some embodiments, the microgel system comprises microgel particlesthat may swell with the organic solvent to form a support material(herein also referred to as an “organogel support material”) forsupporting silicone based ink in a 3D printing operation. As usedherein, “particle” refers to a volume of material that may comprise anetwork of polymers. Depending on the particular embodiment, the swollenmicrogel particles may have a characteristic size at the micron orsubmicron scales. For example, in some embodiments, the swollen microgelparticles may have an average diameter between about 2 and 6 μm, orbetween 0.1 μm and 100 μm, although microgel particles with any suitablesize may be used. Furthermore, an organogel support material may haveany suitable combination of mechanical properties, and in someembodiments, the mechanical properties may be tuned via the relativeconcentration of microgel particles and organic solvent. For example, ahigher concentration of microgel particles in organic solvent may resultin a support material having a higher elastic modulus and/or a higheryield stress.

According to some embodiments, a microgel system may be made frommaterials such that the granular microgel particles undergo a temporaryphase change due to an applied stress (e.g. a thixotropic, “yieldstress” material or “shear thinning” material). Such materials may besolids or in some other phase in which they retain their shape underapplied stresses at levels below their yield stress. At applied stressesexceeding the yield stress, these materials may become fluids or in someother more malleable phase in which they may alter their shape. When theapplied stress is removed, yield stress materials may become solidagain. Stress may be applied to such materials in any suitable way. Forexample, energy may be added to such materials to create a phase change.The energy may be in any suitable form, including mechanical,electrical, radiant, or photonic, etc.

The terms “yield stress” and “yield stress material”, unless indicatedotherwise, should be understood to be a Herschel-Bulkley yield stressdetermined using the Herschel-Bulkley equation

σ=σ_(y) +k{dot over (γ)} ^(p)

where σ_(y) is yield stress, σ is shear stress, k is viscosity index ofthe material, {dot over (γ)} is shear rate, and p is a positive number,and a material having such a yield stress.

Unless indicated otherwise herein, a yield stress of a sample isdetermined by shearing the sample in a rheometer using plate-plategeometry and via the Herschel-Bulkley equation, via the followingprocess. Prior to shearing, the rheometer tool surfaces may be roughenedto prevent or mitigate slipping at the sample-tool interface. Using therheometer, the sample is sheared at a variety of shear rates, extendingfrom high shear rates (e.g., 1000 s⁻¹) to low shear rates (0.001 s⁻¹).For each shear rate, the sample is sheared for 30 seconds, after whichshear stress data is collected and averaged. A series of shear stressmeasurements are collected sequentially for each shear rate. These shearrates are then used, via the Herschel-Bulkley equation, to determine (1)whether the material has a yield stress (i.e., a Herschel-Bulkley yieldstress), and (2) the yield stress for the material. Those skilled in theart will appreciate that, for a material having a yield stress, a plotof shear stress versus shear rate will exhibit a plateau region at lowshear rates, with the data points asymptotically approaching thematerial's yield stress at low shear rates. The yield stress is theshear stress at these low, near-zero shear rates, or an estimate ofshear stress at zero strain rate determined using a low or near-zeroshear rate, such as a shear rate of 10⁻³ s⁻¹. As used herein (unlessindicated otherwise), a “yield stress material” will be a material thathas a yield stress determinable via this process. Those skilled in theart will appreciate that for a yield stress material (i.e., aHerschel-Bulkley yield stress material) at low shear (e.g., a near-zeroshear rate), a shear stress is independent of shear rate and insteadexhibits only a shear stress dependent only on an elastic component ofthe material.

Those skilled in the art that materials having a yield stress will havecertain thixotropic properties, such as a thixotropic time.

As used herein, a thixotropic time is a time for shear stress to plateaufollowing removal of a source of shear. The inventors recognize thatthixotropic time may be measured in different ways. As used herein,unless indicated otherwise, thixotropic time is determined by applyingto a material, for several seconds, a stress equal to 10 times the yieldstress of the material, followed by dropping the stress to 0.1 times theyield stress. The amount of time for the shear rate to plateau followingdropping of the stress is the thixotropic time.

A yield stress material as described herein may have any suitablemechanical properties that, on the one hand, yield under stress toenable injection of an “ink” in a controlled pattern, but, on the otherhand, retain the ink in that patter when the stress is removed. Forexample, in some embodiments, a yield stress material may have anelastic modulus between approximately 1 Pa and 1000 Pa when in a solidphase or other phase in which the material retains its shape underapplied stresses at levels below the yield stress. In some embodiments,the yield stress required to transform a yield stress material to afluid-like phase may be less than about 100 Pa, between approximately 1Pa and 1000 Pa, or between approximately 1 Pa and 10 Pa. Whentransformed to a fluid-like phase, a yield stress material may have aviscosity between approximately 1 Pa s and 10,000 Pa s. However, itshould be understood that other values for the elastic modulus, yieldstress, and/or viscosity of a yield stress material are also possible,as the present disclosure is not so limited.

According to some embodiments, a support material for 3D printing madefrom a yield stress material may allow 3D printing of soft ink materialssuch as silicone to form a desired structure in three dimensions. Forexample, a computer-controlled injector tip may trace out a spatial pathwithin a support material and inject ink materials at locations alongthe path to form a desired 3D pattern or shape. Movement of the injectortip through the support material, with or without vibration, may impartsufficient mechanical energy to cause yielding in a region around theinjector tip to allow the injector tip to easily move through thesupport material, and also to accommodate injection of ink material.However, it should be appreciated that other techniques for impartingenergy to cause yielding in a localized region around the tip of theinjector may alternatively or additionally be used.

After injection, the ink material may be transformed into a permanentsolid-like phase having the printed 3D pattern or shape. In someembodiments, the silicone ink material may be cured within the microgelsupport material using, for example, UV illumination, to achieve a phasechange. The printed object may be extracted after curing by removing thesurrounding support material using any suitable means.

Although printing of ink material using an injector tip is discussed, itshould be understood that the materials and methods disclosed in thepresent application is not limited to any particular 3D printingtechniques.

According to some embodiments, a support material for 3D printing ofsilicone may be immiscible with silicone in the ink material. In someembodiments, it is desirable to reduce the interfacial tension betweenthe support material and silicone to avoid instabilities due to breakupduring printing. In some embodiments, the interfacial tension between anorganogel support material and silicone may be less than about 30 mN/m,or between about 0.5 and 20 mN/m, or between about 0.1 and 5 mN/m.

In some embodiments, a combination of values such as the interfacialtension between the support material and ink material and the yieldstress of the support material may be selected to establish mechanicaland rheological properties between the silicone ink material and supportmaterial such that instability issues such as ink material sag ordispersion during 3D printing of silicone may be alleviated. In someembodiments, the support material—silicone interfacial tension and theyield stress of the support material has a ratio of betweenapproximately 1:5 and 1:20, with a unit of mN/mPa. However, it should beappreciated that other values for the ratio between interfacial tensionand yield stress may enable 3D printing of 3D structures with finefeature sizes. In some embodiments, the materials and methods asdescribed herein may allow 3D printing of silicone with feature size ofless than 200 μm, less than 140 μm, between 10 and 140 μm, between 30and 150 μm, or between 20 and 200 μm.

In accordance with some embodiments, an organic microgel system withsuitable rheological properties may be created through the self-assemblyof di-block and tri-block copolymers in organic solvents and prepared atlow concentrations and proper di-block to tri-block ratios.

In some embodiments, di-block copolymer molecules with a first end and asecond end may be used in forming the microgel. The first end may be arelatively hydrophilic block while the second end may be a relativelyhydrophobic block. In one example, the first end may be styrene and thesecond end may be a rubber such as ethylene/propylene, thus the di-blockcopolymer may be styrene-ethylene/propylene (SEP). The inventors haverecognized and appreciated that when crosslinked, di-block copolymermolecules such as SEP form packed micelles based on polystyrene coresconnected by bridges of ethylene/propylene. In some embodiments, asuspension of discrete micelles is formed in the pure di-block copolymernetwork such as SEP.

In some embodiments, tri-block copolymer molecules with a first end, anintermediate section and a second end may be used in forming themicrogel. The first end and the second end of the tri-block copolymerare relatively hydrophilic, and wherein the intermediate section isdisposed between the first end and the second end and is relativelyhydrophobic. In one example, the first end and the second end may bestyrene and the intermediate section may be a rubber such asethylene/butylene, thus the tri-block copolymer may bestyrene-ethylene/butylene-styrene (SEBS). The inventors have recognizedand appreciated that when crosslinked, tri-block copolymer moleculessuch as SEBS form packed micelles based on hard polystyrene cores and asa result, neighboring polystyrene cores are physically connected bybridges of ethylene/butylene blocks, resulting in a macroscopic network.In some embodiments, the size of each polystyrene core is determined bythe polystyrene block number and mass density within the di-blockcopolymer while the spacing between cores is driven by the swelling ofthe oil-soluble block of the di-block copolymer.

The inventors have recognized and appreciated that di-block copolymersuch as SEP when used alone, may exhibit liquid-like behavior. On theother hand, the inventors have recognized and appreciated that tri-blockcopolymer such as SEBS is a material typically with higher stiffness andhigher yield stress than di-block copolymer. The inventors haverecognized and appreciated that by blending di-block copolymer such asSEP with a tri-block copolymer such as SEBS may lead to improvedmechanical and rheological property to provide support for 3D printingof silicone. Glassy polystyrene cores formed in the blended tri-blockand di-block copolymer system may prevent dynamic exchange of polymersbetween structures. In some embodiments, the mixture of di-blockcopolymer and tri-block copolymer form a jammed pack of microgelparticles or a jammed micro-organogel, with particle size of betweenapproximately 0.1 to 100 μm. As used herein, “jammed” refers to being ina jammed state of a material. The inventors have recognized andappreciated that selection of different types of block units for thedi-block copolymer in the mixture may be used to tune swelling of themicrogel system in organic solvents, while selection of different typesof block units for the tri-block copolymer in the mixture may be used totune the stiffness, among other mechanical and rheological properties ofthe microgel system. In some embodiments, the microgel particle size maybe tuned by the ratio between the di-block and tri-block copolymers aswell as the concentration of the copolymers in the organic solvent. Inone non-limiting example, the ratio between di-block SEP to tri-blockSEBS copolymer is substantially 50:50 (wt %), although it should beappreciated that other suitable ratio may be used.

It should be appreciated that any suitable mixture of block copolymersmay be used to achieve the desired mechanical, rheological andinterfacial properties for 3D printing of soft materials such assilicone. Although linear di-block and tri-block copolymers arediscussed, such discussion are provided as example only and theorganogel support material may be formed with copolymers of any suitableshape, such as linear, stars, crosses.

In some embodiments, to prepare the organogel support material, themixture of di-block and tri-block copolymer molecules are blended in anorganic solvent. The organic solvent may be mineral oil, although anysuitable organic solvent may be used. For example, the organic solventmay be paraffin oil, or any organic solvent immiscible with silicone inkmaterial but can dissolve hydrophobic block units in a copolymer.Subsequently, the blended copolymers and the organic solvent are heatedto a temperature above the glass transition temperature of a block unitof the copolymers in order to melt the copolymer molecules. In oneexample, the organic solvent with the copolymer blend are heated abovethe glass transition temperature of polystyrene. In a non-limitingexample, the blend comprises di-block SEP and tri-block SEBS copolymersand the temperature is between 50 and 150° C., or between 100 and 180°C. It is understood that any suitable amount of heating time and anysuitable mixing or stirring techniques may be used to facilitate meltingof the copolymers, before allowing the organic solvent to cool down tobelow the glass transition temperature of polystyrene when thecopolymers are crosslinked to form a polymer network. In someembodiments, the polymer network comprises microgel particles to be usedas organogel support material in 3D printing of silicone.

In some embodiments, mechanical, rheological and/or interfacial tensionproperties of the organogel support material for 3D printing of siliconeink material may be tuned via the relative concentration of copolymermolecules and organic solvent. For example, a higher concentration ofcopolymers and microgel particles in organic solvent may result in asupport material having a higher elastic modulus and/or a higher yieldstress. In some embodiments, 90% to 99.9%, or 80 to 95%, or more than85% of the support material is organic solvent by weight, although itshould be appreciated that any suitable concentration of the organicsolvent may be used to achieve precision printing of 3D siliconestructure.

In some embodiments the organogel formed by heating a polymer mixture asdescribed above in an organic solvent may, after cooling, be used as thesupport material. In other embodiments, the resulting organogel may befurther processed to remove the solvent, leaving the polymer structureintact. As a result, the microgel particles may take the form of asolid. For example, resulting solid particles may be in the form ofpowders that can be packaged using any suitable means for transport andfor sale, without the necessary accommodations and related costsassociated with an oil-based support material that are mostly liquid. Insome embodiments, the solid particles may comprise a dry polymer networkof crosslinked di-block and tri-block copolymers. The solvent-freeparticles may be reconstituted or re-solvated by a user or customer bythe addition of a suitable amount of organic solvent. In someembodiments, the microgel particles may retain the desired mechanical,rheological and interface tension properties once reconstituted inorganic solvent, without the need for heating. In one example, acustomer may blend the solvent-free microgel particles in mineral oil atroom temperature to obtain an organogel support material for 3D printingof soft materials such as silicone without the capital and energyexpense of heating equipment. However, it should be appreciated thatheating below the glass transition temperature of the polymer mayfacilitate re-solvation.

Turning now to the figures, specific non-limiting embodiments of organicmicrogel systems for 3D printing of silicone and methods for theirpreparation and/or use are described in more detail.

FIG. 1 depicts a schematic representation of one embodiment of anapparatus for printing an ink material in a support material. In theembodiment shown in FIG. 1, a computer-controlled injector tip tracesout a horizontal path with a lateral velocity of ν towards the rightdirection within a support material while injecting ink materials atlocations along the horizontal path. The injected ink materialsolidifies to form a desired 3D pattern or shape. The support materialfluidizes as movement of the injector tip displaces the support materialand imparts sufficient mechanical energy to cause yielding in a regionaround the injector tip to allow the injector tip to easily move throughthe support material. In some embodiments, movement of the injector tipmay additionally include a vibratory movement in order to locallyfluidize the yield stress support material with the vibration energy.The yielding also accommodates injection of ink material.

FIG. 2 is a schematic diagram depicting printing of ink material in asupport material comprising di-block copolymers and another supportmaterial comprising tri-block copolymers. The scale bars are 15 nm. InFIG. 2, di-block copolymer materials form packed micelles with fluidicyielding properties. Printing into the di-block micelle suspension ismade difficult by its fluid-dominated rheology. Printed features maymove under buoyancy forces or break up into droplets. FIG. 2 also showsthat with respect to crosslinked tri-block copolymers, stiff network ofcrosslinked micelles may be formed. In some cases, printing into thetri-block copolymer network is made difficult by its solid properties.While the crosslinked network is capable of supporting structures, theprinting nozzle severs polymer bridges as it traverses through theprinting medium, irreversibly damaging the gel when the copolymermaterial is not yielding enough to provide elastic recovery in the wakeof the injector needle.

FIG. 3 is a schematic diagram depicting printing of ink material in asupport material comprising a blend of di-block copolymer SEP andtri-block copolymer SEBS in mineral oil. The scale bar is 15 nm. FIG. 3shows that a jammed microgel may be formed by crosslinking both thedi-block and tri-block copolymer, where it is understood that tri-blockcopolymer may contribute to the overall stiffness of the microgel whilethe di-block copolymer may contribute tuning the swelling property ofthe microgel in mineral oil. By blending di-block SEP copolymer withtri-block SEBS copolymer, improved mechanical and rheological propertymay be reached as support for 3D printing of silicone such that thesupport material fluidizes at the location of applied stress and rapidlyre-solidify upon the removal of stress to enable precise printing of 3-Dstructures with a feature size of less than 200 μm, or less than 100 μm.

According to one example, a microgel comprising SEP and SEBS copolymersis prepared by mixing in light mineral oil (NF/FCC) (Fisher Scientific)a blend of Kraton G-1702, which is a linear SEP di-block copolymer, withKraton G-1650, which is a linear SEBS tri-block copolymer. The mixtureof block copolymers is prepared at 2.25 wt % di-block copolymer, 2.25 wt% tri-block copolymer, and 95.5 wt % light mineral oil. The mixedsamples are heated to 150° C. and continuously mixed using a ScilogexOverhead Stirrer set to 250 RPMs for 4-6 hours. Although 150° C. isprovided in this example, any suitable temperature higher than the glasstransition temperature of the polystyrene block in the copolymers may beused.

Although a 50:50 ratio between di-block and tri-block copolymer byweight and 95.5% wt % light mineral oil is disclosed, such formulationis provided by way of an example only and the materials and methodsaccording to aspects of the present application are not limited to anexact formulation. The properties of the mixture solution may vary withthe mineral oil concentration. When the mineral oil concentration is toolow, formation of a macroscopic gel is observed whereas when the mineraloil concentration is too high, the solution does not behave as anelastic solid at low stresses and is thus not suitable for 3D printing.In some embodiments, the organogel support material may be prepared withmore than 90%, more than 94%, less than 99%, less than 97%, between 94%to 99%, or between 94 wt % to 97 wt % mineral oil. Similarly, theproperties of the mixture solution may vary with the di-block:tri-blockcopolymer ratio. When the di-block:tri-block ratio is too high, thesolution no longer behaves as an elastic solid, whereas when thedi-block:tri-block ratio is too low, the solution behaves as amacroscopic gel. In some embodiments, the organogel support material maybe prepared with a di-block:tri-block copolymer ratio of less than75:25, less than 55:45, more than 45:55, more than 25:75, between 25:75to 75:25, or between 45:55 to 55:45 by weight.

To illustrate the mechanical and rheological behavior of the supportmaterial according to some embodiments with different formulations, FIG.4 is a data plot of measured elastic and viscous shear moduli G′ and G″for samples prepared with only tri-block copolymer (curves 410, 412),with a 50:50 mixture of di-block and tri-block copolymer (wt %) (curves420, 422) and with only di-block copolymer (curves 430, 432) as afunction of an applied oscillatory stress frequency at low strain rateamplitude (1%) for microgels prepared according to some exemplaryembodiments. In the data plots in FIG. 4, the copolymer blends aredissolved in mineral oil with an oil concentration of 95.5% (wt %).

FIG. 5 is a data plot of measured shear stress σ as a function of shearrate c for microgels prepared according to some exemplary embodimentswith a 10:90 wt % mixture of di-block and tri-block copolymer (curve512), with a 50:50 wt % mixture of di-block and tri-block copolymer(curve 514) and with only di-block copolymer (curve 516). To identifythe yield stress of the material, stress was measured underunidirectional shear at different shear-rates. The yield stresscorresponds to a plateau in the shear stress at low shear-rate and isobtained through a Hershel-Bulkley curve fit. The rheological behaviorof the copolymer system can be tuned. The data in FIG. 4 shows that asthe concentration of tri-block polymer is increased relative to di-blockpolymer, the yield stress and stiffness of the gel increases. The datain FIG. 5 shows that unrecoverable yielding of the 100% tri-blockmaterial is observed in shear-rate sweeps; when the applied shear stressexceeds the yield stress of the gel, SEBS tri-block bridges are severed.Conversely, materials with high di-block proportions behave like viscousliquids, exhibiting a crossover of elastic and viscous shear moduli infrequency-sweep measurements, and having no observed yield stress inshear-rate sweeps (FIGS. 4, 5). The moduli and the yield stress of thedifferent formulations can also be controlled by changing the globalpolymer concentration by adjusting the mineral oil concentration.

FIG. 6 is a data plot of measured shear rate c as a function of recoverytime after yielding for microgels prepared according to some exemplaryembodiments. In this experiment, the rate of elastic recovery in thematerial after the removal of applied shear stress is determined. Thethixotropic time was measured by first applying a shear stress greaterthan the yield stress of the organic microgel system. The applied stressis then dropped below the yield stress of the material and the shearrate is measured as a function of recovery time. The recovery timerepresents the thixotropic time which is the duration over whichshear-rate drops to zero after a high level of applied shear stress israpidly removed. A short thixotropic time improves printing performancebecause it reduces the duration over which the support material isfluidized and mechanically unstable. The data plot in FIG. 6 shows shearrate as the microgels transition from sheared to non-sheared regime. Forrecovery time of less than 1 second, the shear rate is above zero anddecreases as time elapses and the microgels are still in the fluidizedregion of the injector nozzle.

Desirable organogel support materials, according to some embodiments,may have thixotropic times less than 2.5, less than 1.5 seconds, lessthan 1 second, or less than 0.5 seconds, and greater than 0.25 secondsor greater than 0.1 seconds. Because of the yield stress behavior of theorganogel support materials, materials deposited into the supportmaterial (such as through 3D printing techniques described herein) mayremain fixed in place in the yield stress material, without the yieldstress material needing to be cured or otherwise treated to reverse aphase change (e.g., by heating to cross-link, following printing).Rather, the yield stress materials permit an indefinite working time ondeposition of materials inside yield stress materials, includingprinting of objects within yield stress materials. For example, inexperiments described herein, printed objects stayed in place formultiple months (i.e., greater than one minute, greater than one hour,greater than one day, greater than one week, and greater than one month)without movement following deposition, without curing of the yieldstress material or otherwise without treatment to reverse a phasechange. This may permit continuous printing/deposition of a materialwithin a yield stress support material for greater than one minute,greater than one hour, greater than one day, greater than one week, orgreater than one month, where continuous printing would not includeceasing for reasons related to the support material (e.g., treating orcuring) but may include ceasing for reasons related to printing (e.g.,reloading of materials to be printed or movement of print head to adifferent location).

The data in FIGS. 4-6 show that the 50:50 di-block and tri-block polymerblend has an elastic modulus of about 100 Pa. This formulation has ayield stress of 3-4 Pa and a thixotropic time of about one second. Sucha combination of rheological behaviors is the fingerprint of jammed softgranules.

FIG. 7 is a phase contrast microscopy image of a microgel preparedaccording to an exemplary embodiment and diluted in mineral oil. In thisexample, the microgels were diluted by adding neat mineral oil to dilutethe sample to 1% of the original copolymer concentration, thus allowingthe microgel particles to suspend in the mineral oil in an un-jammedstate. The microscopy image in FIG. 7 shows a plurality of granularparticles 2 to 4 μm in size indicating the formation of multiplemicrogel particles in the mineral oil solution. Each particle mayinclude a volume containing a polymer network, where the polymer networkcomprises one or more hard micelle cores formed by crosslinkedpolystyrene.

FIG. 8 is a data plot of measured small angle X-ray scattering (SAXS)intensity as a function of wave vector q for microgels preparedaccording to some exemplary embodiments. The SAXS data in FIG. 8provides an indication of the spacing between the micelle cores in thepolymer network.

Small angle X-ray scattering measurements on organogel samples preparedat a global polymer concentration of 4.5% consisting of 100% di-blockcopolymer, 100% tri-block copolymer or an equal blend of the two. Bydetermining the location of the first peak (q₁), one is able tocalculate the average core to core spacing (d) for each sample, d=2π/q₁.With the core to core spacing, one can estimate the volume of each unitcell as =4πd³/6. The polymer chain concentration is defined as thenumber of polymer chains per total volume of the system and can becalculated as

${C_{chain} = \frac{\frac{N_{av}}{M\; W_{p}}}{\frac{1}{\rho_{p}} + \frac{C_{o}}{p_{o}C_{p}}}},$

where C_(o) is the concentration of the mineral oil, ρ_(o) is thedensity of the mineral oil, C_(p) is the concentration of the blockcopolymer, ρ_(p) is the density of block copolymer, MW_(p) is themolecular weight of the block copolymer. By multiplying the polymerchain concentration by the volume of each unit cell, we are able toestimate the number of copolymer chains present in a single core of theorganogel system. We find the pure tri-block organogel consists of ˜12tr-iblock chains/unit cell core; the pure di-block organogel consists of˜6 di-block chains/unit cell core; and the blended organogel consists of˜6 tri-block chains and ˜3-4 di-block chains/unit cell core.

FIG. 9 is a confocal microscopy image of printed silicone featuresaccording to some embodiments. In the example in FIG. 9, printing ofsilicone elastomers into the organogel support medium is accomplishedusing a custom made 3D printer consisting of a modified linear stage asa syringe pump attached to three linear translational stages. The threelinear translational stages follow a predetermined trajectory at aspecified translational speed while the syringe pump continuouslydeposits the silicone elastomer into the organogel medium at a set flowrate Q. To test the level of control provided by the micro-organogel forsilicone 3D printing, w a series of linear features are created at manydifferent combinations of nozzle translation speed ν and injection flowrate. A test ink is made from vinyl terminated polydimethylsiloxane(PDMS) polymer, mixed with (methacryloxypropyl)methylsiloxane-dimethylsiloxane copolymer crosslinking agent at a 3:1ratio. To enable 3D imaging with a laser scanning fluorescence confocalmicroscope, fluorescent beads are dispersed in the PDMS ink beforeprinting. The images in FIG. 9 show side-on and end-on projections ofprinted silicone features printed using different nozzle velocity ν withstraight and smooth features that are nearly round in cross-section.Notably, the cross-sectional images in the x-z plane show that theprinted features are approximately circular and are substantiallysymmetrical in width and height, which indicate that the amount ofinterfacial tension between the ink material and support material islow, similar to Pickering emulsion.

FIG. 10 is a data plot of measured printed feature area A as a functionof material flow rate and tangential nozzle velocity. Although a fewselected samples are shown, any suitable flow rates may be selected fromthe range between 1 ul/hr to 3000 ul/hr. Nozzle velocities may be 1 mm/sto 10 mm/s, or 0.1 mm/s to 1 m/s. The data in FIG. 10 is a quantitativeanalysis of the 3D stacks as shown in FIG. 9 and illustrates a positivecorrelation between the printed feature size with √{square root over(Q/ν)}, where Q is the material flow rate and ν is the nozzle tangentialvelocity along the x-direction in FIG. 9. In other words, the faster thenozzle moves, the smaller the feature size; the higher the material flowrate, the larger the feature size. The results are consistent withA=Q/ν, and the principle of conservation of volume, where A is thefeature's cross-sectional area. The images in FIG. 9 show that featurewidth and feature height are also nearly the same across all flow ratesand translation speeds. Thus, features between about 1 mm and 100 μm indiameter can be created by following the simple volume conservation. Insome embodiments, features as small as 30 μm were generated by reducingthe ink flow rate and increasing the nozzle translation speed,indicating the high-level of precision for printing silicone structures.

FIG. 11A is a photograph showing measurement of interfacial tensionusing the pendant drop method. FIGS. 11B-11E are data charts showinginterfacial tension measurements for the interfaces measured using thependant drop method, according to some embodiments. To estimate thestrength of interaction between the organogel support material and thePDMS ink in the absence of added particulate fillers, a series ofinterfacial and surface tension measurements were performed. Surfacetension measurements are calculated using the pendant drop method inwhich a drop is suspended from a needle into air. Images of the dropletare taken and analyzed in MATLAB to determine the interfacial tensionbetween the drop and air. Interfacial tension measurements betweensilicone oil and mineral oil are taken using the liquid substratemethod, in which a drop of light mineral oil (NF/FCC)(Fisher Scientific)is placed on a bath of 100 cSt silicone oil (Sigma Aldrich). Photoanalysis and mass measurements were used to determine the spreadingparameter of the droplet while interfacial tension between silicone oiland mineral oil using the spreading parameter and the individual surfacetensions were determined from the pendant drop method.

In order to define the interface between the drop and surroundingmaterial, an image of the drop is captured in front of a diffused lightsource (FIG. 11A). The location of the drop edge is determined byfitting an error function to the intensity of the image as we move outradially from the center of the image (FIG. 11B). Once defined, the edgelocation is converted to Cartesian coordinates and we fit a curve over arange of points to smooth the shape of the drop, determine the radius ofcurvature, and determine the angle formed with the x-axis (FIG. 11C). Wecalculate the interfacial tension for each location along the dropletusing the Young-Laplace equation, 1/(R/a)+sin φ/(x/a)=−B(x/a)+2 where Ris the radius of curvature at location (x,z), a is the radius ofcurvature at the origin (0,0), sin φ is the angle between the tangent tothe drop at x,z and the x axis and B is defined as B=a²gΔρ/γ where g isthe gravitational constant, Δρ is the difference in density between thedrop and the support bath, and γ is the interfacial tension. Toeliminate outlying values resulting from extreme solutions to theYoung-Laplace equation (e.g. when x=0), we plot the calculatedinterfacial tension values from smallest to largest and remove theextreme values from consideration (FIG. 11D). The average of theremaining values is calculated to determine the interfacial tensionbetween the drop and the support bath (FIG. 11E).

The measurement results show γ_(s)=19.0±0.9 mN/m for silicone oil in airand γ_(m)=29.9±1.5 mN/m for mineral oil in air. To determine theinterfacial tension between silicone oil and mineral oil, γ_(sm), theliquid substrate method is used in which a drop of mineral oil is placedon a silicone oil substrate. The thickness of the spread mineral oillayer, h, is determined from measurements of its diameter and volume. Abalance of gravitational and interfacial forces yields ½{tilde over(ρ)}gh²=γ_(m)−γ_(s)−γ_(sm), where {tilde over(ρ)}=ρ_(m)(ρ_(m)−ρ_(s))/ρ_(s) is the reduced density of the two-oilsystem, ρ_(s) is the silicone oil density, ρ_(m) is the mineral oildensity, and γ_(ms) is the interfacial tension between silicone oil andmineral oil. Using the parameters for γ_(s) and γ_(m) obtained fromsurface tension measurements described above, the interfacial tension isfound to be γ_(ms)=12.2±1.2 mN/m.

In order to achieve stability in printed structures, the instabilitiesarising from interfacial tension must be balanced by the yield stress ofthe liquid like solid. To determine the minimum feature size for a givenyield stress, print a range of low viscosity silicone oil are printedinto the mineral oil organogel with varying yield stresses and featurestability was observed over time. We find that minimum stable featuresize decreases with increasing yield stress while the time till breakupis dependent on the viscosity of the printed material. While thereremains interfacial instabilities between silicone oil based ink and themineral oil based support material, the printing of fine, precisionsilicone structures is possible when the curing time of the silicone isless than the time in which the interfacial instabilities arise.

In some embodiments, the interfacial tension between silicone inkmaterial such as PDMS and the surrounding organic support material mayalso assists in creating 3D printed parts with smooth surfaces. After 3Dprinting linear features made from Momentive UV Electro 225 PDMS andcrosslinked through UV curing, the cured part is removed from theorganogel support material, cleaned by serially washing in solvents andsurfactant solutions and characterized. FIGS. 12A-12C show scanningwhite light interferometry measurement results demonstrating that theprinted parts have a surface roughness of 150 nm. FIG. 12D is a scanningelectron microscopy image of the cross-section of a printed siliconestructure demonstrating the uniformity of printed structures, while FIG.12e is a scanning electron microscopy image of a 3D printed “dog-bone”part designed for testing mechanical strength. The structures aspictured in FIG. 12E are printed, cured, and removed from the organicsupport material. FIG. 12F is a data plot showing extensionalstress-strain tests on dog-bone samples demonstrating excellentmechanical integrity of 3D printed silicone parts which fail atapproximately 700% strain

To demonstrate the range of 3D printing capabilities enabled by themicro-organogel system, a variety of silicone elastomer structures withvarying size, complexity, and materials have been printed. FIGS. 13A-13Gshows photographs of 3D printed silicone structures using microgelsystem as support material according to some embodiments. FIG. 13A showsa model trachea implant printed out of a room temperature vulcanizationsilicone, Mold Max 10, in which the diameter of the tube fluctuates withheight. After letting the Mold Max 10 silicone cure at room temperaturefor 24 hours, the model can be removed from the organogel supportmaterial and handled. In another example, FIGS. 13B-13D show isometric,top and side views, respectively, of a printed silicone scaffold with asinusoidal wave pattern in the x-y direction as well as the x-zdirection. The silicone scaffold structure has a dimension of 20×20×8 mmand is printed out of Momentive UV Electro 225 silicone with featuresize on the order of 250 μm. To demonstrate the ability to print complexstructures for biomedical applications such as complex macro- tomeso-scale vasculatures, FIGS. 13E-13F show 3D printing of a UV curingsilicone elastomer ‘sea anemone’ into the organogel support material.The printed structure transitions from one large base tube with a 25 mmdiameter to six smaller vessels with 3 mm diameters. Once the structureis cured, removed from the micro-organogel support material and cleaned,fluids such as water may be pumped through all twelve openings at highflow rates as shown in FIG. 13G.

While the present teachings have been described in conjunction withvarious embodiments and examples, it is not intended that the presentteachings be limited to such embodiments or examples. On the contrary,the present teachings encompass various alternatives, modifications, andequivalents, as will be appreciated by those of skill in the art.Accordingly, the foregoing description and drawings are by way ofexample only.

What is claimed is:
 1. A method of manufacturing a three-dimensionalsilicone structure, the method comprising: injecting ink materialcomprising silicone into a support material and displacing the supportmaterial with the ink material, and curing the silicone material whileretaining the ink material with the support material, wherein: the inkmaterial is injected in a pattern conforming to the three-dimensionalstructure, and wherein, the support material comprises an organicsolvent and a block copolymer such that the support material isimmiscible with the ink material.
 2. The method of claim 1, wherein aninterfacial tension between the support material and the ink material isless than 30 mN/m.
 3. The method of claim 2, wherein the interfacialtension is between 0.5 and 20 mN/m.
 4. The method of claim 2, whereinthe interfacial tension is between 0.1 and 5 mN/m.
 5. The supportmaterial of claim 1, wherein the block-copolymer comprises a network ofparticles with an average diameter of between 2-6 μm.
 6. The method ofclaim 1, wherein the block-copolymer comprises a network of particleswith an average diameter between 0.1 μm and 100 μm.
 7. The method ofclaim 1, wherein the support material is a shear thinning material. 8.The method of claim 7, wherein the support material has a yield stressof less than 100 Pa.
 9. The method of claim 7, wherein the supportmaterial has a yield stress of between 1 and 10 Pa.
 10. The method ofclaim 7, wherein the support material has an interfacial tension toyield stress ratio of between 1:5 and 1:20 mN/mPa.
 11. The method ofclaim 1, wherein the block copolymer comprises a di-block copolymer. 12.The method of claim 11, wherein the di-block copolymer is styreneethylene/propylene (SEP).
 13. The method of claim 1, wherein the blockcopolymer comprises a tri-block copolymer.
 14. The method of claim 13,wherein the tri-block copolymer is styrene ethylene/butylene styrene(SEBS).
 15. The method of claim 1, wherein the pattern conforming to thethree-dimensional structure has a minimum feature size of 140 μm. 16.The method of claim 1, wherein the pattern conforming to thethree-dimensional structure has a minimum feature size of 30 μm.
 17. Asupport material for supporting silicone-based ink in a 3D printingoperation, the support material comprising: a plurality of microgelparticles, each of the plurality of microgel particles comprising acrosslinked polymer network, wherein the crosslinked polymer networkcomprises: a plurality of tri-block copolymer molecules; a plurality ofdi-block co-polymer molecules; and an organic solvent, wherein theplurality of microgel particles are swollen in the organic solvent. 18.The support material of claim 17, wherein the organic solvent is mineraloil.
 19. The support material of claim 17, wherein each of the tri-blockcopolymer molecules has a first end, an intermediate section and asecond end, wherein the first end and the second end are hydrophilic,and wherein the intermediate section is disposed between the first endand the second end and is hydrophobic.
 20. The support material of claim19, wherein each of the di-block copolymer molecules has a first end anda second end, and the first end is hydrophobic and the second end ishydrophilic.
 21. The support material of claim 20, wherein the di-blockcopolymer and tri-block copolymer contain styrene.
 22. The supportmaterial of claim 18, wherein the microgel particles have an averagediameter of between 2-6 μm.
 23. The support material of claim 17,wherein an interfacial tension between the support material and thesilicone-based ink is less than 30 mN/m.
 24. The support material ofclaim 17, wherein an interfacial tension between the support materialand the silicone-based ink is between 0.1 and 5 mN/m.
 25. The supportmaterial of claim 17, wherein the microgel particles are in a jammedstate.
 26. A method of manufacturing a support material for supportingsilicone-based ink in a 3D printing operation, the method comprising:blending a di-block copolymer and a tri-block copolymer in an organicsolvent; heating the solvent mixture to above a first temperature;cooling the solvent mixture to below the first temperature to form aplurality of microgel particles.
 27. The method of claim 26, wherein theorganic solvent is mineral oil.
 28. The method of claim 27, wherein 90%to 99% of the solvent mixture is mineral oil by weight.
 29. The methodof claim 27, wherein the ratio between the di-block copolymer and thetri-block copolymer is between 45:55 and 55:45 by weight.
 30. The methodof claim 27, wherein: the di-block copolymer and tri-block copolymercontain styrene; and the first temperature is the glass transitiontemperature of polystyrene.
 31. The method of claim 26, wherein thefirst temperature is between 50 and 150° C.
 32. The support material ofclaim 26, wherein the microgel particles have an average diameter ofbetween 5-6 micrometers.